Production of cold refrigerant gas



June 5, 1962 J. sT s E 3,037,360

PRODUCTION OF COLD REFRIGERANT GAS Filed May 17. 1960 4 Sheets-Sheet 1 I it: I D 40" I v I 1 20 I, I

INVENTORS JESSE B. STARNES PAUL E. LOVEDAY RICHARD R. CARNEY A 7'TORNEV June 5, 1962 J. B. STARNES ET AL 3,037,360

PRODUCTION OF cow REFRIGERANT GAS Filed May 17, 1960 4 Sheets-Sheet 5 INVENTORS JESSE B. STARNES PAUL E. LOVEDAY RICHARD R. CARNEY A TTORN Y June 5, 1962 J. B. STARNES ET AL PRODUCTION OF cow REFRIGERANT GAS 4 Sheets-Sheet 4 Filed May 17, 1960 lNVENTORS JESSE B. STARNES PAUL E. LOVEDAY RICHARD R. CARNEY A T TORNE States Unite York Filed May 17, 1960, Ser. No. 29,628 10 Claims. (Cl. 62-97) This invention relates to a method of and apparatus for producing a cold refrigerant gas, and more specifically to a system for producing a cold gaseous refrigerant in selectable quantities and of selectable temperature.

Cold refrigerant gases are widely used for treating materials. For example, the physical strength of welded honeycomb panels such as the type disclosed in U.S. Patent No. 2,849,591 to Fullerton et al. is greatly benefited by low temperature treatment after welding. Also, cold refrigerant gases may 'for example be employed in chilling metal parts prior to shrink fitting, freezing and refrigerating perishable products such as foods or biologicals, ad in low temperature grinding operations.

The prior art has proposed numerous systems for supplying cold refrigerant gases. For example, a high pressure refrigeration circuit employing a closed fluid stream such as Freon could be employed, but the required equipment has been found to be prohibitively expensive.

Another proposal has been to introduce an air stream into a liquid refrigerant scrubber, but such an arrangement only provides a gas mixture at one temperature level. Industrial needs dictate that a practical cold refrigerant gas generation system be capable of producing gas at selectable :fiow rates and selectable temperatures. For this reason, the scrubbing arrangement would also require auxiliary means for varying the gas temperature, and such means entail additional complicated equipment and expense.

A principal object of this invention is to provide an improved system for producing cold refrigerant gas in selectable quantities and at selectable temperatures.

Another object is to provide an improved cold refrig erant gas generation system which is simpler and more efiicient than the heretofore proposed systems.

These and other objects and advantages of this invention will be apparent from the following description and accompanying drawings in which:

FIGURE 1 is a schematic flow diagram of a system for producing a cold refrigerant gas according to the present invention;

FIGURE 2 is an enlarged front elevational view of a hydraulically expanded assembly which is rolled to form the internal passageways of the heat exchanger in FIG- URE 1;

FIG. 3 is an enlarged partial isometric view of the FIGURE 2 assembly;

FIGURE 4 is an elevation-a1 view taken partly in cross section, of a heat exchanger suitable for employment in the FIGURE 1 system;

FIGURE 5 is an end view of the heat exchanger taken along the line 5-.-5 of FIGURE 4;

FIGURE 6 is an end View of a heat exchanger similar to that shown in FIGURES 4 and 5, but modified to afford countercurrent instead of concurrent flow;

atent air is provided at ambient temperature and at a desired flow rate, the gas containing at least one condensable impurity such as carbon dioxide as an atmospheric contaminant. A source of low-boiling liquefied gas refrigerant, e.g. nitrogen, is also provided and the compressed gas is cooled and at least partially cleansed by indirect heat exchange with the liquid refrigerant. The heat exchange is continued for suflicient duration to completely vaporize the liquid refrigerant, and the cooled, cleaned gas is mixed with the vaporized refrigerant so as to form the cold refrigerant gas at desired volume and temperature.

In a preferred embodiment, the compressed gas stream is supplied at a first higher pressure, reduced to a second lower pressure, and thereafter throttled sufficiently to obtain critical flow before cooling with the liquid refrigerant. The expression critical flow as used herein refers to the gas flow quantity existing when the absolute pressure ratio in the throttling step reaches a certain maximum value, which on further increase in such ratio does not further decrease the mass flow rate.

It will be noted that one of the important features of a preferred embodiment of this invention is the employment of liquid nitrogen as the refrigerant, and recovery of its latent and sensible refrigeration by indirect heat exchange with the compressed air. One could vaporize liquid nitrogen by atmospheric heat and then superheat the resultant nitrogen gas also by external heat to the desired temperature for the refrigerant gas. However, this procedure would be extremely inefficient because over one-half of the available refrigeration in the liquid nitrogen is discarded to the atmosphere. Any system which utilizes external heat to vaporize a low temperature liquefied gas and warm the resultant vapor to the desired temperature for refrigerant purposes can deliver a volume of refrigerant gas only equivalent to the volume of liquefied gas consumed. In the present method, the latent heat of the liquid nitrogen is employed to cool and clean a large volume of another gas, air, which is then employed to augment the total volume of delivered refrigerant gas. Assume, for example, that a consumer requires 1,000 cu. ft. N.T.P. per hr. of refrigerant gas at F. A simple liquid nitrogen vaporizing and superheating system using,

atmospheric heat would require 1,000 cu. ft. N.T.P. per hr. of liquid nitrogen for this purpose. In marked contrast, the present invention requires only 250 cu. ft. N.T.P.

er hr. liquid nitrogen, the remainder of the gas volume being provided as low cost raw air.

In the present invention, liquid nitrogen is uniquely suited as the source of refrigeration in view of its inertness, cleanliness, and low boiling point of 320 F. at atmospheric pressure. Other refrigerants do not possess all of these characteristics. For example, solid carbon dioxide has been considered and discarded since its sublimation temperature is about 109 R, which imposes a severe limitation on this compound as a refrigerant at the lO0 'F. level. The driving force for heat transfer (AT) would be very small at the end of the refrigeration step, and the sensible refrigeration available in the refrigerant gas at this temperature level is also very small. I

Although nitrogen is the preferred low-boiling liquefied gas refrigerant, air is the preferred compressed gas, and the invention will be specifically described in terms of these components, it is to be understood that other fluids may be used in the invention. For example, nitrogen, helium, neon or argon would be suitable for use as the compressed gas component, and any liquefied gas having a boiling point below about K. may be employed as the liquefied gas refrigerant, for example, argon, helium or neon. If the particular intended use for the cold refrigerant gas necessitates an inert fluid, a combination of nitrogen liquid and compressed nitrogen gas may be ada vantageously employed in the practice of this invention.

Referring now more specifically to the drawings and FIGURE 1, liquid nitrogen is stored in thermally insulated container 10. This container may, for example, be of the type disclosed in US. Patent No. 2,725,722 to P. M. Ahlstrand et al., or in US. Serial No. 599,733 filed July 24, 1956 in the name of P. E. Loveday et al., now Patent No. 2,951,348. Container is provided with suitable safety devices to prevent excessive pressure buildup therein, as for example, bursting disk 11 and gas relief valve 12. Also, a source of compressed nitrogen gas may he provided and introduced to the container 10 through conduit 13 having regulator valve 14 therein. The compressed nitrogen gas serves to generate suflicient pressure in container 10 to force the liquid out through withdrawal conduit 15 and control valve 16 therein.

If container 10 is to be portable and thus service several stationary heat exchangers, or is to be removed for recharging with liquid nitrogen, it may be connected to heat exchanger assembly 17 by charging conduit 18. The latter is connected at opposite ends to withdrawal conduit 15 and delivery conduit 19 by couplings 20, and contains safety valve 21 to prevent excessive pressure buildup when the nitrogen flow path is dead-ended in heat exchanger 17. The liquid nitrogen flow is controlled by means of valve 22 in delivery conduit 19, the latter also containing pressure gage 23 and safety valve 24. Liquid nitrogen delivery conduit 19 passes through the outer walls of heat exchanger 17 and terminates in passageway 25.

Compressed air containing normal atmospheric impurities is introduced through conduit 26, cleaned of solid particles in filter 27, and reduced in pressure to an intermediate value such as 60 p.s.i.g. in regulator 28. Pressure gage 29 provides a visual indication of the pressure downstream of regulator 28, and the air stream is further throttled to valve 30 sufliciently to obtain critical flow across such valve. The pressure downstream of valve 30 may be on the order of 22 p.s.i.g., and due to the critical flow feature, further reduction of pressure would not produce an increase in the mass flow rate. The low pressure air, preferably at about 22 p.s.i.g., is then passed through flow-indicating rotameter 31 to heat exchanger assembly 17, the temperature and pressure of such gas being visually indicated by thermometer 32 and gage 33, respectively. By maintaining a constant intermediate pressure after valve 28, and maintaining critical flow across valve 30, the air flow rate is made independent of pressure fluctuations which may occur within the heat exchanger assembly 17 or the air supply piping.

Heat exchanger assembly 17 comprises a casing 34 which is thermally insulated by a layer of low-conductive material 35 such as polyurethane foam, so as to reduce the heat in leak and loss of refrigeration to the atmosphere. Although the heat exchanger may be any type affording intimate thermal contact between the nitrogen refrigerant and the cooling air, a pair of coiled and elongated thermally conductive plates disposed in spaced contact to form a cellular structure, have been found particularly suitable. These plates are illustrated schematically in FIGURE 1 as forming nitrogen passageway 25, and will be described later in detail. The vaporizing nitrogen in passageway 25 and the cooling air flow cocurrently in thermal association from one end to the other end of heat exchange assembly 17 in the space bounded by casing 34 and longitudinal baffle 36, whereupon the flows are reversed and the two streams pass in the opposite direction through the space boundedby the opposite side of baffle 36 and second longitudinal baflle 37 for further heat exchange. Finally, the flow directions of the two heat exchanging fluids are again reversed for end-to-end passage through the space bounded by the opposite side of second longitudinal bafile 37 and casing 34. Safety valve 37a is provided to relieve any abnormal pressure buildup Within casing 34.

A valve 38, preferably of the extended stem type, is provided at the discharge end of the nitrogen passageway 25, the latter fluid having been vaporized and superheated in such passageway. The nitrogen flow rate is held substantially constant by the uniform pressure within the liquid nitrogen supply container 10 and by a fixed setting of valve 38. On discharge of cold nitrogen gas from valve 38 into the surrounding space, mixing occurs between the cold nitrogen gas and the partially cooled and cleaned air, thereby producing the desired cold refrigerant gas. The latter is then discharged from heat exchanger assembly 17 for use as desired through conduit 39 extending through the casing wall. The temperature of the cold refrigerant gas may be visually observed by reading gage 40.

As previously discussed, the nitrogen and air flow rates are preferably maintained constant, so that the discharge temperature indicated by gage 40 will not vary significantly despite possible pressure fluctuations. One of the important advantages of the invention is that extreme flexibility in cold refrigerant gas flow rates and temperatures is achieved in a simple and reliable manner. For example, if a relatively colder refrigerant gas is desired this may be achieved by further opening the nitrogen discharge valve. On the other hand, if a warmer refrigerant gas is needed, valve 38 would be closed an appropriate amount. If a relatively larger quantity of refrigerant gas is to be provided at the same temperature, either or both regulator 28 and valve 30 are opened to increase the .air flow and valve 38 is further opened to increase the nitrogen flow a corresponding amount.

Since the circulating air Will always be cooled below the dew point of water and often below the dew point of carbon dioxide, these atmospheric contaminants will be continuously deposited on the various surfaces inside casing 34. Continuous operation of the heat exchanger assembly 17 Without shut down would eventually result in excessive pressure drop on the air side due to deposited water ice and solid carbon dioxide. However, duplicate assemblies may be provided and operated alternately with the off-stream assembly being thawed and prepared for reuse. If desired, only the colder portion of the heat exchanger assembly need be duplicated for alternate operation; the remaining major portion of the exchanger freezes out a minor part of the impurities and is capable of extended operation without thawing. To this end, drain conduits 41 are provided for removal of water from the casing.

As shown in detail in FIGURES 2 and 3, the preferred nitrogen conduit comprises a pair of elongated conductive sheets and 51, preferably of aluminum or other heat conductive metal, disposed in spaced contact to form a cellular structure having therebetween a labyrinth of internal passages 52, 53, 54, and 56 for nitrogen flow. These passages may be formed by placing flat sheet 5-5 upon flat sheet 51, and bonding together in suitable manner the marginal edges and registering areas 57, illustrated herein the form of rectangular and square buttons, and hydraulically expanding the areas adjacent thereto to form walls 50 and 51 of the internal passageways. This expansion can be accomplished by providing inlet and outlet connections opening into the unbonded areas between the sheets 59 and 51, and through which hydraulic fluid is introduced for expanding such areas to provide passages. The internal passages formed thereby may be of varying shape and dimension through the length of conduit 25 to suit the character of the fluid flowing in the passages at each point therein. The passages adjacent the liquid inlet are preferably of the type shown in FiGURE 3 which permit low and approximately equal flow resistance in either vertical or horizontal directions. Such pattern is conducive to good distribution of the liquid across the width of the vaporizer so that the operating load is uniform.

Above the liquid distributing section containing passages 52, the passages are preferably formed as long,

vertical channels 53 comprising the vaporizing section. Such passages are advantageous in promoting a priming action wherein the fluid with increasing vapor content accelerates upward through the channels and by its turbulence brings the fluid in good heat exchange relation with the conduit walls.

As the nitrogen fluid emerges from the upper ends of vaporizing channels 53, any remaining unvaporized liquid will either be disengaged from the vapor in passages 54 above, or will be reduced to tiny vapor-borne droplets or mist. The passages 54 forming the separator section are somewhat similar to passages 52, providing low flow resistance in vertical and horizontal directions favorable to disengagement of remaining liquid. Passages 54 also provide much extended wall surfaces on which any disengaged liquid is held and vaporized.

A vapor superheating section above the separator section contains long vertical passages 55 similar to vaporizing channels 53. If desired, passages 55 may be interrupted by one or more horizontal passages 58 for assuring good distribution of the fluid among the superheating channels.

Above the superheating section, a gas collecting section is formed of passages 56 similar to passages 52. The warmed gas leaving passages 55 flows upwardly and horizontally to the withdrawal connection 59, and the horizontal flow must be accompanied by very low pressure difference across the width of the heat exchanger so as not to upset the flow distribution.

It will be noted that the vaporizer section (passages 53) and superheating section (passages 55) of FIGURE 2 are fore-shortened. These sections provide the majority of the heat transfer into the gas material and comprise the major portion of the heat exchanger length.

FIGURES 4 and 5 show an assembly preferred as the heat exchanger 17 of FIGURE 1, although other types of heat transfer surface could be employed, for example, externally finned tubes or smooth tubes, metalbonded to sheets of metal. However, the FIGURES 25 construction is preferred from the standpoints of convenience, compactness, portability and low cost. The FIGURE 2 and 3 sheet assembly is preferably coiled into a spiral 70 around a center pipe core 71, and the spiral assembly is positioned inside an inner pressure container consisting of cylindrical side walls 72 and welded end closures 73. Outer cylindrical casing 74 with end closures '75 form a space 76 for insulation around the inner container.

Liquid nitrogen is introduced through conduit 77 into the internal passageways 25 .at the outer edge of the spiral heat exchanger. After flowing through the exchanger to the inside edge of the spiral, the resulting cold nitrogen gas leaves the passageways 25 through outlet connection 78 joining discharge valve 38. This valve is positioned in side the center pipe core 71 and it discharges cold nitrogen gas into such core.

Two radial partitions 79 are positioned across the insulation space 76 and are longitudinally sealed to the inner wall of the outer casing 74 and the outer wall of the inner container 72 so as to form an inlet compartment 80 for the warm air entering through conduit 19. This compartment is devoid of insulation and the air is distributed uniformly along the length of the exchanger by means of holes 81 drilled in the container wall 72 within the air inlet compartment 80. The inlet air flows spirally around the nitrogen passageway heat transfer surfames 25 toward the center of the exchanger and enters the center pipe core 71 through opening 81a, cut in its wall. Here, theair mixes with the vaporized nitrogen released into the same space through valve 38, and the resulting cold refrigerant gas exits from the exchanger through conduit 82.

First drain conduit 83 terminating in inlet air compartment 80 is provided for removing water which would otherwise condense in the first 180 of the air fiow path and collect in such compartment. Second S-shaped drain conduit 84 connects with the inner container 72, and drains water condensing in the next 360 of the air flow path. Additional drain conduits may be provided if desired.

Although the air and nitrogen streams have been illustrated and described as being heat exchanged cocurrently, countercurrent flow is also practicable. The latter offers certain advantages since the air would be cooled to a much lower temperature by the nitrogen and would therefore be considerably cleaner with respect to carbon dioxide. Also, liquid water may be first condensed and drained oif so that the life of the frost zone is longer. Simultaneously, the nitrogen is warmed to a substantially higher temperature by countercurrent heat exchange, and subsequent mixing would provide the same degree of operating flexibility as with cocurrent heat exchange. One advantage of cocurrent exchange is that the discharge temperatures of the mixed streams are somewhat easier to control and maintain at desired levels.

FIGURE 6 illustrates how the heat exchanger of FIGURES 4-5 may be arranged for countercurrent rather than cocurrent heat exchange. That is, the positions of the nitrogen inlet and outlet connections are interchanged so that liquid nitrogen is introduced to the innermost spiral through conduit 85 and discharged as gaseous nitrogen from the outermost spiral through connection 86 for mixing with cooled air to form the cold refrigerant gas.

FIGURE 7 shows an alternative heat exchanger assembly 90 which could be substituted for the assembly 17 of FIGURE 1. That is, the assembly within the dotted lines of FIGURE 1 may be replaced with assembly 90. Referring now more specifically to the latter, casing 90a contains two nitrogen passageways 91 piped in parallel, which may be of the partially bonded, hydraulically expanded type illustrated in FIGURES 2 and 3. Low pressure air is introduced downwardly through opening 92 in the top of the casing 90a, and flows in countercurrent indirect heat exchange with vaporizing liquid nitrogen rising through either of passageways 91, the latter having been introduced through conduit 19 at the base of the assembly. The warmed vaporized nitrogen is withdrawn from the top of conduits 91 into a manifold 93, and directed through conduit 94 having flow control valve 95 therein. Meanwhile the cold, cleaned air leaves the bottom of casing 90a through conduit 96, and is joined by the vaporized nitrogen from conduit 94 for mixing and formation of the cold refrigerant gas. It will be appreciated that the desired quantities and proportion of air and nitrogen may obtained by the same procedure as previously described in conjunction with FIGURE 1. Furthermore, although illustrated in FIGURE 7 in the vertical position, this is not essential and the horizontal or up-ended positions are suitable and would assist drainage of water from the exchanger.

FIGURE 8 illustrates still another alternate heat exchanger assembly 97 similar in appearance to that of FIGURE 7, except that the vaporized nitrogen stream is not withdrawn from the heat exchanger for external mixing. Instead, it is released directly from the upper ends of passageways 91 into the entering air stream and returns downwardly on the shell side of the casing 98 in heat exchange with vaporizing nitrogen inside the passageways 91. The performance of the FIGURE 8 heat exchanger differs slightly from that of FIGURE 7 in that the clean nitrogen dilutes the carbon dioxide and water content of the entering air and therefore tends to reduce the amount of impurity deposition on the heat transfer surfaces. This will prolong the operating life of the heat exchanger between thawout periods, but is also increases the amount of carbon dioxide present in the resultant cold refrigerant gas. Furthermore, liquid nitrogen control valve 99 is required in the nitrogen inlet conduit 19 rather than the discharge conduit from the heat exchanger.

spar pee Although the preferred embodiments have been described in detail, it is contemplated that modifications of the method and the apparatus may be made and that some features may be employed without others, all within the spirit and scope of the invention as set forth herein.

What is claimed is:

l. A method for producing a cold refrigerant gas at desired, selectable volume and temperature which comprises the steps of supplying compressed air at ambient temperature and at a desired flow rate, such air containing at least carbon dioxide as an atmospheric contaminant; supplying liquid nitrogen refrigerant; cooling and at least partially cleaning said compressed air by indirect heat exchange with said liquid nitrogen; continuing such heat exchange for sufficient duration to completely vaporize said liquid nitrogen; mixing the cooled and cleaned air with the vaporized nitrogen so as to form said cold refrigerant gas at desired volume and temperature.

2. A method according to claim 1 wherein the compressed air stream is supplied at a first higher pressure, reduced to a second lower pressure, and thereafter throttled sufiiciently to obtain critical flow before cooling with said liquid nitrogen.

3. A method according to claim 1 wherein the flow of vaporized nitrogen mixing with the cooled air is adjustable.

4. A method according to claim 1 wherein the indirect heat exchange between the compressed air and vaporizing nitrogen is cocurrent.

5. A method according to claim 1 wherein the indirect heat exchange between the compressed air and vaporizing nitrogen is countercurrent.

6. A method for producing a cold refrigerant gas at desired, selectable volume and temperature which comprises the steps of supplying compressed air at a first pressure, ambient temperature and a selectable flow rate, such air containing at least carbon dioxide as an atmospheric contaminant; supplying liquid nitrogen refrigerant; reducing the pressure of said compressed air to a second lower pressure, and thereafter sufficiently throttling such lower pressure air to obtain critical flow; cooling and at least partially cleaning the throttled air by indirect heat exchange with said liquid nitrogen; continuing such heat exchange for sufiicient duration to completely vaporize said liquid nitrogen; adjusting the fiow of the vaporized nitrogen, and mixing the cooled and cleaned air with the flow adjusted, colder vaporized nitrogen so as to form said cold refrigerant gas at desired volume and temperature.

7. Apparatus for producing a cold refrigerant gas at desired, selectable volume and temperature including means for supplying compressed air at ambient temperature and at adjustable flow rates; means for supplying liquid nitrogen refrigerant; means for passing said compressed air and said liquid nitrogen in sufficient indirect heat exchange so as to cool the air and simultaneously completely vaporize the liquid nitrogen; and means for mixing the cooled air with the vaporized nitrogen so as to form said cold refrigerant gas.

8. Apparatus according to claim 7 in which means are rovided for reducing the pressure of the compressed air, means for throttling the reduced pressure air sufficiently to obtain critical flow across such means, and means for passing the throttled air to the indirect heat exchange means.

9. Apparatus according to claim 7 in which means are provided for adjusting the dew of vaporized nitrogen before passage to said means for mixing with the cooled air.

10. Apparatus for producing a cold refrigerant gas at desired selectable volume and temperature including means for supplying compressed air at a first pressure, ambient temperature and at adjustable fiow rates; means for supplying liquid nitrogen refrigerant; means for reducing the pressure of said compressed air to a second lower pressure; means for sufficiently throttling the reduced pressure air to obtain critical flow across such means; means for passing the throttled air and said liquid nitrogen in sufficient indirect heat exchange so as to cool the air and simultaneously completely vaporize the liquid nitrogen; means for adjusting the flow of such vaporized nitrogen to a desired quantity; and means for mixing the flow adjusted and vaporized nitrogen with the cooled air so as to form said cold refrigerant gas.

References Cited in the file of this patent UNITED STATES PATENTS 2,685,181 Schlitt Aug. 3, 1954 2,922,286 Rae Jan. 26, 1960 2,926,501 Morrison Mar. 1, 1960 2,951,346 Collins et a1 Sept. 6, 1960 

1. A METHOD FOR PRODUCING A COLD REFRIGERANT GAS AT DESIRED, SELECTABLE VOLUME AND TEMPERATURE WHICH COMPRISES THE STEPS OF SUPPLYING COMPRESSED AIR AT AMBIENT TEMPERATURE AND AT A DESIRED FLOW RATE, SUCH AIR CONTAINING AT LEAST CARBON DIOXIDE AS AN ATMOSPHERIC CONTAMINANT; SUPPLYING LIQUID NITROGEN REFRIGERANT; COOLING AND AT LEAST PARTIALLY CLEANING SAID COMPRESSED AIR BY INDIRECT HEAT 